Does anyone know how to tell if that cabbage at the store is from this year? Most of the cabbage I see seems to be from last year's harvest. It makes bland cole slaw and bland Danish Red Cabbage - Rødkål (Roedkaal).

Cabbage is not in season now so it's likely that it was harvested earlier this year or last year, depending on where it's from. It's a cool-weather plant and lasts all winter here. Maybe you could ask the grocery store manager about it.

__________________The trouble with eating Italian food is that five or six days later you're hungry again. ~ George Miller

I'm just guessing here, but I bet any of the cabbages in the stores right now are "winter cabbages". They keep better than the early season variety, and they are sturdier (tougher) so they can keep over the winter. I made a batch of coleslaw with a newly-purchased cabbage a couple weeks ago. I still have some of it in the refrigerator, waiting to see if it will ever wilt down and soften! Toughest coleslaw we have ever had.

Coleslaw tip:
I love it!
Make some form of it every week.
I rough chop the cabbage and a small sweet onion or a quarter of a sweet onion and the carrots I have already rough chopped and pulsed in the food processor Into a cold water bath to which I've added a couple T's of Kosher salt and sugar which have been melted in a little hot water.
Let the cabbage/carrots/sweet onion sit in the water for only about fifteen minutes. Rinse well and pat dry or use a salad spinner. At this point I put the cabbage/carrot/onion in the food processor and pulse until I have the consistency I want.
Then into a bowl>sprinkle of sugar>enough japanese mayo to coat everything. A T of homemade mustard. Season. Into fridge overnight.
Read this.The Food Lab: How To Make the Best Creamy Cole Slaw | Serious Eats

The other thing is to stand the head of cabbage in water overnight. The stored cabbage dries out. This helps to re-hydrate it. What is in season right now is the loose-leaf/open head cabbage. I've only seen it in the green variety and that is what is in season in the garden right now.